The Intelligence betrays the Leviathans, using its pawns to slaughter them for their genetic material to create Harbinger, the first Reaper.
The Intelligence constructs an army of "pawns" to gather genetic information throughout the galaxy.
After observing that their subservient races create synthetic races who consistently and violently rebel, the Leviathans create an Intelligence to seek a way to permanently preserve organic life.
The Leviathans consider themselves the galaxy's first and only apex race.
The Leviathans dominate the Milky Way, enthralling every new sapient race they encounter and protecting them in return for tribute.